The utilization of neutrons for contact-less or non destructive measurement has been well documented. Many techniques have been proposed and reduced to practice. The ongoing development of sensor technology such as semi conductor material based electronic circuit elements with a reproducible transducer function in response to ionizing radiation will certainly yield further inventions and improvements of sensor devices and sensor systems. Such technologies reveal the energy spectrum of impinging corpuscular or photonic “particles”. Analysis of the energy spectra can reveal the elemental and isotopic bulk composition of the object irradiated by the neutrons. The analysis has to subtract background spectra, accounting spatial factors, and then compare the remaining spectrum against known reference spectra as is practiced by the general discipline of spectroscopy.
Neutron based analysis techniques include but are not limited to:
Prompt Gamma Neutron Activation Analysis PGNAA
Pulsed Gamma Neutron Activation Analysis PGNAA
Neutron Elastic Back Scatter NEBS
Pulsed Fast Neutron Activation Analysis
Instrumental Neutron Activation Analysis
Thermal Neutron Activation Analysis
The source of the neutrons may be a nuclear fission reactor which is a very expensive option; radionuclide decay where neutrons are emitted by an isotope such as Californium 252; charged particle accelerator machines that target protons or deuterons onto a target which holds certain low atomic number elements in order to yield neutrons from fusion or spallation reactions; compact sealed versions of the accelerator method; sealed devices where a deuterated or tritiated solid target is hit by deuterium or tritium ions extracted from a plasma and accelerated toward the target surface; laser powered devices that cause sufficient transient pressure and heat in tiny droplets to induce fusion reactions; and electrostatic devices referred to as inertial electrostatic confinement (EC) or inertial electrostatic fusion (IEF) or “fusor” where a solid target is not required as the fusible species are confined to a zone of increased collision probability.
Nuclear fusion grade collisions of low atomic number nuclei have been utilized to produce particles such as neutrons, protons and various fragments of the nuclei that constitute the fused product.
Typical examples of such fusion reactions for neutron generation include:
a) 1D2+1D2→2He3 (0.82 MeV)+0n1 (2.45 MeV)
b) 1D2+1D2->1T3 (1.01 MeV)+1p1 (3.02 MeV)
c) 1D2+1T3->2He4 (3.5 MeV)+0n1 (14.1 MeV)
d) T3+T3 reaction yielding neutrons of various energies for a “white” spectrum.
d) protons on Be
Proton generating reactions may utilized.
The rate of neutron generation for commercially available neutron generators may be expected to have an upper limit of approximately 1010 n/s for the DD reactions a) and b). In the case of the DT reaction c) the increased collision cross-section is known to yield a factor of between 80 and 100 more neutrons per unit time for the same operating conditions or reactor type. As the reaction rate is increased there is a reduction of operational life between servicing sessions. The devices that are classed as sealed tube neutron generators have lifetimes of a few hundred hours. Neutron generators with distributed solid targets and target sputter erosion effects mitigation means have been developed to have lifetimes of 3000-4000 hours with DD neutron outputs of 108 and 106 neutrons per second from point like origins respectively for devices developed and manufactured by Thermo and Sodern respectively. The plasma-gas target neutron generator as disclosed in patent application Sved WO03019996 incorporated herein by reference which has been developed and manufactured by NSD-Fusion by has specific performance in the 107 DD n/s range for a 50 mm long electrode which may be increased by length and input power increase. It has an operational lifetime between servicing that is measured in years. The desire for higher neutron output must be tempered by the implications for neutron shielding and the consequential size and cost impact on any practical neutron applications system housing.
There is a relationship between the flux of impinging neutrons and the time required to accumulate spectral statistics with sufficient information content. Flux is usually stated in terms of neutrons per square centimeter per second. A unit volume of matter will be mostly transparent to the neutrons with exceptions determined by the probability of capture of a slow neutron by a nucleus or collision with deflection of the fast neutron trajectory and an energy exchange. These interactions may be modelled mathematically and hence simulated in computer programs to yield predictions about the overall behavior of a flux field of neutrons with defined initial conditions. By such means one experienced in the skills of nuclear physics may design or optimize design parameters so that the sensors will receive the maximum flux of scattered neutrons if that is the intention or emitted gamma energy domain photons with mitigated flux of unwanted radiation.
Such analysis and reductions to practice may be compromised or constrained by available materials and equipment. Neutron energies from sources are usually determined by the physics of nuclear reactions so that initial energies are essentially fixed at certain values as indicated in the table. Some nuclear reactions can cause a spread energy spectrum which can be utilized. Certain techniques will moderate or slow the neutrons by collisions. These resultant slow neutrons will then be more easily captured by nuclei which then decay, sometimes in compound chains of decay steps, to yield distinctive energy photons.
The design process of a neutron interrogation system considers such factors and uses published characteristics for the various nuclear interactions. Also to be incorporated into the design process are the unwanted but present secondary emissions from shielding and instrumental limitations and sources of electronic noise. Another design factor is the counting limitation of detector systems where the frequency of gamma or neutron events that can be correctly resolved and processed into electronic event energy information will impose a practical upper limit on the impinging flux of neutrons.
This disclosure is concerned with design parameters that can be influenced by the neutron source emission zone topology. In particular the length, width, thickness and segmentation of the source emission zone within a sealed unit. Such design parameters plus the specific performance of the neutron emission zone can give the neutron interrogation system designer more flexibility to achieve a system performance specification such as effective measurement of objects being put through the system at a specified rate.
The necessity for this invention arises from industrial applications of neutron interrogation including but not limited to:
A) security screening of luggage where only 8 seconds per item is specified for Level 1 or 2 screening.
B) food safety screening on the production line where the food units may be packaged, pre-packaged, raw food stuff delivered to the processing line, frozen food units such as individual vegetables, animal carcasses, cuts of meat, pulp vegetable matter, fruit juice or any other variant of bulk food or a stream of food units.
C) Postal letters at a mail sorting facility
D) Couriered packages at a sorting facility
E) Bulk material in a production line flow on a conveyor belt, channel or in a pipe
A common characteristic in the above mentioned examples is that the flow rate of items to be scanned is relatively high and indeed too high for the neutron interrogation systems that have been deployed into practical operation. It must be clarified that the on-line analyzer neutron interrogation systems that can be found in cement, coal and some other mineral mining and processing operations monitor a fast moving conveyor belt that carries bulk material that has a high uniformity where compositional changes may vary only relatively slowly from the point of view of the process operator.
Line sources characteristics have been recognized and exploited with radionuclides since at least the 1960s with descriptions such as line or bar or rod or annular source. http://www.orau.org/ptp/collection/Sources/sources.htm
Neutrons have been applied to the interrogation of materials to help determine the structure and elemental or isotopic composition of the materials. The observable interaction products are well documented as gamma photon energy spectral signature data under categories of application such as Neutron Activation Analysis, Thermal Neutron Analysis and Prompt Gamma Neutron Activation Analysis, Inelastic Neutron Scattering to name only a few of the common terms. In these applications it is necessary to have a flux of neutrons that can pass into the material that is to be interrogated. The source of the neutrons may be a nuclear fission reactor which is a very expensive option; radionuclide decay where neutrons are emitted by an isotope such as Californium 252; charged particle accelerator machines that target protons or deuterons onto a target which holds certain low atomic number elements in order to yield neutrons from fusion or spallation reactions; compact sealed versions of the accelerator method; and various plasma producing devices where ions are extracted and accelerated by electrostatic fields to impinge on solid targets or self collide within the plasma.
Neutrons are attractive for many industrial analysis, medical and security inspection applications because they are very penetrating and can have residual non-biological side effects that are so small that they cannot be measured by any known technology. Unlike X-rays and gamma rays they will interact with other atoms/isotopes of materials which will in turn produce a gamma photon energy signature. For example, neutrons will interact with nitrogen which is the major constituent of all chemical explosives. The neutron interacts with the nitrogen nucleus, which in turn gives off a characteristic energy gamma photon. A standard commercially available gamma spectrum detection system can find concealed explosives in baggage and it is very difficult to shield out the neutrons without raising suspicion. Enhancement of the gamma detection and recognition electronics and software can enable a commercially attractive explosives detection system to be feasible if a compatible neutron source can be integrated into a sufficiently compact, robust, long lived and safe system.
From the industrial customer point of view, whilst neutrons are attractive they are hard to produce on demand conveniently or cheaply. The radionuclide source of neutrons has been the lowest cost source of neutrons but the inability to switch off the source has imposed difficult operational safety requirements. Solving these design requirements for practical neutron applications systems has added to the cost of manufacture and the cost of operation. In the case of the frequently used Californium 252 neutron source, there is a half life of 2.65 years. This requires the operator of an industrial measurement system to top-up the 252Cf neutron source every two and a half years if a reduction of fifty percent of the neutron flux can be tolerated. There is a recurring cost associated with procurement, safety monitoring and ultimate authorized disposal of the radioisotope neutron source capsules.
An alternative technology that has been offered commercially for about 30 years is the so-called sealed tube neutron generator. This technology is based on the trigger device on H-bomb nuclear weapons where an intense flux of neutrons is generated to initiate the chain reaction. Typically, a sealed tube neutron generator uses an electric field to accelerate deuterium ions from an appropriate source into a tritium target so as to bring about a fusion reaction with accompanying neutron generation. A typical sealed tube neutron generator uses a deuterium-tritium fusion reaction to yield 14.1 MeV energy neutrons. The shelf life of such a device must be very long and is constrained by the 12 year half life of tritium. The operational life of the device is very short prior to its total destruction within the nuclear weapon. These sealed tube devices have been commercialized but have found almost no acceptance in industrial settings where the requirement for economic life cycle costs have been too severe for that technology. Either 252Cf neutron sources have been most often used or there has been no implementation of a neutron interrogation system. The industrial user would rather select another non-neutron technology after trading the advantages and disadvantages.
The sealed tube neutron generators have an operational life or endurance that is typically only a few hundred hours. Recent products on the market claim 2000 hours or even 4000 hours before tube replacement. The cost of exchanging of a sealed tube device, which includes authorized recovery of the tritium gas, is prohibitively high. 252Cf neutron sources can still compete against conventional sealed tube neutron generators. If the conventional sealed tube neutron generators could demonstrate an operational lifetime of 10,000 hours or better still 25,000 hours, there would be less of a cost difference compared to 252Cf neutron sources. However the routine tube replacement cost would have to be comparable to or less than the 252Cf top-up cost.
A further problem with the sealed tube neutron generator is that the obtainable lifetime is achieved by a combination of compromises. The fusion reaction rate is reduced from the maximum that the device can theoretically deliver by a reduction of the applied ion beam current. This has the desired effect of reducing the sputter erosion rate of the solid target. This may be described as de-rating the device output performance in order to lengthen the device operational life. A consequence is that the neutron production rate is also reduced. A typical specified neutron production rate for applications in on-line minerals analysis is 1×108 neutrons per second total output. Californium 252 sources of 2×108 n/s are utilized. After 2.65 years the 252Cf source strength will have reduced to 1×108 n/s. Therefore a sealed tube neutron generator has to deliver at least 1×108 n/s to be comparable. The costs of manufacturing have forced sealed tube neutron generator manufacturers to modify existing weapons-derived devices. Consequently, a deuterium-tritium isotope fusion reaction has to be utilized to achieve the 1×108 n/s reaction rate. If a deuterium-deuterium fusion reaction were to be used, the same sealed tube device, filled only with deuterium, would emit about 1×106 n/s.
A further problem with the sealed tube neutron generators that utilize the D-T fusion reaction is that the D-T fusion neutron energy is 14.1 MeV. The Californium 252 decay neutrons range in energy from about 1 MeV to 10 MeV. The mean energy is about 2.1 MeV. There are also various gamma photons emitted as part of the decay products. While D-T 14 MeV neutrons are useful because they are more penetrating than 2.1 MeV neutrons, and while the higher energy enables certain interactions with certain elements such as oxygen, there are also problems. In particular, there are neutron interactions that rely on low energy or so-called thermal energy neutrons where the neutron has been slowed down to kinetic energies amounting to tens of KeV or less. In order to slow the high energy neutrons, moderator materials are used to cause energy reduction through collisions. The released energy is manifested as other energy forms such as gamma photons. In some applications these photons act as unwanted detected noise that masks the desired gamma photons from thermal neutron interactions. Therefore it is often necessary to use source neutrons of as low an energy as practical. So 252Cf neutrons of mean energy 2.1 MeV would be preferred over 14.1 MeV D-T neutrons. Clearly in this example a mono energy 2.45 MeV D-D neutron generator would be more preferred if other requirements would be compatible.
In some applications, such as Boron Neutron Capture Therapy (BNCT) for the treatment of inoperable cancer tumors, the neutron flux must be precisely defined in order to be generally accepted as part of an approved medical therapy. The quality of the neutron flux from accelerator sources or sealed tube neutron generators is deemed by some commercially organized BNCT researchers not to be ideal. Accelerator spallation neutron sources will generate a range of neutron energies. The moderation and collimation of a range of neutron energies is obviously more difficult than for mono energy neutrons. Sealed tube devices do provide mono energy neutrons but suffer from unreliability of the neutron output as do the accelerator spallation neutron sources. The solid targets that these devices use suffer from altered characteristics due to the damage they incur through use. The electrostatic confinement of fusible ions in a neutral gas and ion mix plasma does not suffer from target degradation. The reactants are continuously renewed in the fusion zone. Reactant gas contamination can be mitigated so that the neutron output quality can be constant for a given set of controllable operating parameters.
An advantage that any electrical neutron generator should offer is the ability to switch on and off repeatedly to create a pulsed mode of operation. The pulse mode duty cycle may range from minutes or seconds of ON time and similar intervals of OFF time to micro- and even nanoseconds pulse duration It is not necessary, cost effective or perhaps not practical to offer the entire pulsing duty cycle range in every neutron generator. Pulsed current where the current is many orders of magnitude greater than for a DC or chopped DC pulse mode mandates pulsed operation. The main advantage of the pulsing mode is to cut off the noise caused by the higher energy neutron interactions and then detect the thermal neutron or other delayed interactions where prompt gamma photons are not emitted instantaneously. The implementation of mechanical shutters to make 252Cf into a pseudo-pulsed neutron source is not practical nor cost effective.
A further cost consideration is the often requested configuration of the neutron source as a non-point of origin. The neutron flux intensity decreases as the inverse square of the distance from the point of origin in the case of a point-like source. In many industrial applications the object that is to be interrogated by the neutrons has a large characteristic size. For example, a mineral stream on a conveyor belt may be so wide that the extremities will receive a significantly reduced flux of neutrons compared to the middle portion. This has undesirable consequences for the efficiency of the detection of gamma photons arising from the neutron interactions. Optimization of the gamma detector configuration may not be sufficient or cost effective. A second point source neutron emitter is often introduced to mitigate the edge losses.
With the usage of two point sources, there is a doubling of the cost of the neutron source. This is tolerated for some commercial neutron analysis systems where 252Cf has been the only practical source. However, it is more problematic for a neutron application system that would use two neutron generator devices. Since each individual neutron generator system consists of the reactor device, a high voltage power subsystem, an electronic controller sub-system and ancillary cooling sub-system, multiple copies of the equipment would be required. The operation of two or more sealed tube devices connected in parallel to one set of appropriately specified ancillary sub-systems seems to be only a marginal cost reduction compared to two separate sealed tube neutron generator sets.
A more ideal line source that is made from discrete 252Cf radioisotope neutron sources can be contemplated. Such a linear geometry 252Cf neutron source has been proposed for land mine detection. In such a configuration 252Cf pellets would be spaced in a line at intervals optimized so that the gamma detectors would not suffer from the non-uniformity of the neutron flux field. The length of the land mine detection linear neutron source that would necessarily be suspended in front of a suitable vehicle may be as much as 4 meters. This implies that many 252Cf pellets would be used. A design constraint that can be expected is that the total activity level should remain low enough to gain a permit for operation. This may reduce the effectiveness of the land mine detector system. Higher radioactivity levels may not be permitted without severe design requirements for the withdrawal and shielding of the numerous capsules of 252Cf. Damage to the system by an exploding land mine is a further difficulty. Neutron generators seem more appropriate since they can be instantly switched off. However, the use of several conventional sealed tube neutron generators would be a significant fraction of the total system cost. Further complexity would arise from the control problem associated with matching of output of multiple sealed tube units. A sealed vessel containing a linear solid target and a particle beam deflection system can be envisaged. The particle beam would be deflected much like electrons in a cathode ray tube or television picture tube to scan back and forth along the target. The neutron emission would exit the device as a moving spot or point source. Such a system is considered to be a very expensive option due to the inherent complexity and reliability concerns. A long linear configuration neutron generator reactor chamber in accordance with embodiments of the present invention is expected to overcome such difficulties.
The sealed tube neutron generator technology is inherently age-limited by the unavoidable erosion of the solid target. This component is a metal such as titanium that has been impregnated with tritium or deuterium gas. The incident high energy deuterons have the effect of causing sputter erosion of the target. The sputter product condenses as a metallic film on the inside surfaces of the sealed tube device. The use of voltages near 100 kilovolts results in a short circuit condition as the metallic film builds up. Even before this ultimate failure mode, the highly localized beam causes a hot spot and associated gas depletion within the target. Various neutron yield degradation mitigation schemes have been employed but the fact remains that the best guaranteed lifetime of a sealed tube neutron generator is only 4000 hours.
The invention Sved WO03019996 incorporated herein by reference has been conceived as a new technology that addresses the cost problems associated with 252Cf and the short life neutron generators as described above. The elimination of solid target erosion is an attractive possibility of the Inertial Electrostatic Confinement IEC concept. The constant renewal of the colliding nuclei within the fusion target zone is a further attraction for producing a constant neutron yield. The demonstration and experimentation in three different laboratories of a different linear geometry electrostatic nuclear fusion reactor of the type described by Gu et al. US2003223528 which is not the present invention has been an additional incentive to create a practical system for commercial industrial application.